Climate Science Glossary

Term Lookup

Settings

Use the controls in the far right panel to increase or decrease the number of terms automatically displayed (or to completely turn that feature off).

Term Lookup

Term:

Settings

Beginner Intermediate Advanced No DefinitionsDefinition Life:

All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

Posted on 24 April 2011 by Ari Jokimäki

There have been claims that cosmic rays could have contributed significantly to the global warming over the past century. According to a new study, that is not the case. Instead, during the last 50 years, cosmic rays seemed to have caused warming of only about 0.002°C - a negligible amount compared to observed warming.

Cosmic rays have been claimed to be a significant source for the formation of cloud condensation nuclei, and through that mechanism they have been claimed to affect Earth's climate significantly. There have been many studies debunking these claims, and currently it seems that the possible effect of cosmic rays on the climate is small (see Skeptical Science advanced rebuttal to cosmic ray hypothesis). It is however likely that there are some mechanisms by which cosmic rays do affect cloud formation at least a little. For example, recently a correlation was found in Europe between diurnal temperature range (difference between daily maximum and minimum temperatures) and strong cosmic ray flux changes (Forbush decreases and ground level enhancements).

A new study by Erlykin et al. evaluates the effects of cosmic rays to cloud cover. The paper discusses previous cosmic ray research, and performs some new analysis. They use cloud cover measurements of International Satellite Cloud Climatology Project (ISCCP). There have been criticisms of ISCCP data, but the researchers believe that ISCCP problems do not affect their results.

Evidence of cosmic ray effects

Solar activity changes during the sunspot cycle and effects Earth's temperature. According to Erlykin et al., the change in temperature then effects the height of low clouds. The change of height makes some of the low clouds migrate to the medium cloud region. This would then cause a decrease in the amount of low clouds. This would also be a candidate for the origin of the correlation between cosmic ray flux and low cloud cover (because solar activity changes are known to affect also cosmic ray flux). When analysing cloud cover changes it would make sense to look at the changes in both the low and medium clouds. When both low and medium clouds are included to the correlation analysis, the correlation with cosmic ray flux is poor.

Forbush decreases (few percent decrease in cosmic ray flux that lasts few days) have been claimed to cause cloud cover changes. However, the observed correlations between cloud cover changes and cosmic ray flux during a few Forbush decreases could have been coincidental. Additionally, delay between the changes in cosmic ray flux and in cloud cover seems to have been too long for the two to be causally related. Some of the strongest Forbush decreases have also been associated with strong changes in solar activity, which further clouds the cosmic ray warming hypothesis.

Nevertheless, there is some evidence relating to Forbush decreases that suggests some kind of effect on cloud cover. The strongest effect seems to lie in the stratosphere, but there is also some evidence for an effect in troposphere. It is unclear if these effects are due to cosmic rays or solar activity.

For cosmic ray flux ground level enhancements there also is some evidence of cloud effects. The strongest effect seems to occur in the poles which might suggest genuine cosmic ray involvement. Erlykin et al. estimate a global effect of about 1% in cloud cover during ground level enhancements.

In a recent analysis, it was found out that in mid-latitudes rapid changes in cloud cover are associated with changes in cosmic ray flux and in surface temperature. Strong reaction here only occurs during the rapid cloud cover changes, which are rare events, so the total effect of cosmic rays still seems to be small. Here too it is unclear if the observed effects are due to cosmic rays or simultaneous changes in solar activity.

In the case of these rapid changes, the rates of change in clouds and in cosmic rays have been different, which speaks against the cosmic ray origin. On the other hand, linking these cloud changes to solar activity is also problematic, because most of the UV radiation stops in the stratosphere, and therefore would not be able to cause changes to most of the cloud cover. It is possible that the explanation for cloud changes is found in surface temperature changes caused by solar activity changes.

There is lot of evidence from polar regions on stratospheric cosmic ray effects, which usually are related to solar flares. It seems that cosmic rays have an effect on stratospheric aerosols, ozone, wind, temperature, pressure and ionisation. There is also some evidence of possible effect on stratospheric clouds during Forbush decreases and other cosmic ray flux changes, but the size of the possible effect is unclear.

Correlation between cosmic ray flux and cloud cover

Analysis of cloud cover correlation between cosmic rays and UV radiation reveals that correlation is significant only in these cases: low cloud cover with cosmic rays (positive correlation) and with UV radiation (negative), medium cloud cover with UV radiation (positive), and high cloud cover with cosmic rays (negative). For the case of UV radiation, the different sign for the correlation with low and medium cloud cover might be explainable by part of low cloud cover changing to medium cloud cover when surface temperature changes. For cosmic rays, the negative correlation with high cloud cover is not what would be expected from ionization mechanism.

The correlation of cosmic rays with low cloud cover is the correlation that has been claimed to cause the global warming. However, the spatial distribution of the correlation does not fit to the expectations from cosmic ray origin. Correlation is strong in mid-latitudes, but weak in the poles and in the equator. Mid-latitude correlation is about 8 times higher than the correlation in equator and in poles. This distribution of correlation results a possible effect to global cloud cover which is smaller than 1%.

According to Erlykin et al., the cosmic ray flux has decreased about 0.6% in last 50 years. Assuming 1% effect of the cosmic rays to cloud cover, this would cause a warming of 0.002°C in global surface temperature. This is negligible compared to the observed global warming in last 50 years.

In conclusion, while cosmic rays do seem to have some minor atmospheric effects, they do not seem to have anything to do with global warming, particularly over the past century.

Comments

I suspect cloud formation has a major influence over our climate. Even the IPCC admitted that cloud formation is poorly understood. Cosmic rays may not be the main driver, but a deeper understanding of the mechanisms involved would surely be a significant step forward.

If I simply stand outside and feel the difference between full sun, and cloud, and just feel the difference in energy reaching Earth's surface, I can't help thinking this is what it's all about.

I bought Svensmark’s book because I like to expose myself to both sides of an argument and I wanted to get the “best” of the pro-GCR side. When I pointed out to other readers (on Amazon) how biased the book’s review of scientific literature was I was truly amazed at how quickly I was “voted down” (I never even got into Calder’s (the real author) dubious history of scientific reporting).

I admit that it is an interesting hypothesis, but only folks highly selectively biased by personal belief could possibly believe that galactic cosmic rays have a climate effect greater than that of the change we have seen in atmospheric CO2 levels. Little in the scientific literature supported it then and even less does now.

Mmmm, two possible explanations for global warming one with poor to no correlation and no theoretical underpinning, the other with clearly understood physics and extremely good correlation. How to choose, how to choose, what a dilemma.

"The well-known Forbush Decreases in CR intensity (denoted‘FD’ and typically 3% for 2 or 3 days) caused by changes in the solar wind - and attendant CR modulation - following solar ‘eruptions’, should, if the CC, CR correlation is causal, give rise to CC reductions. Indeed, even if not causal but if both CR and CC changes are due to a third variable (solar irradiance, for example) then a correlation should result. Svensmark et al. (2009) have claimed such a correlation for CR FD and the liquid cloud fraction (LCF), but this has been disputed (by us, Laken et al.,2009 and by Calogovic et al.,2010)."

muoncounter: "Who did this analysis? Again, which way are the changes?"

muoncounter: "Isn't this the best take-away? The contribution of CR to ‘climate change’ is quite negligible."

Erlykin et al. conclude:

"The increase in temperature predicted is 0.002°C, a value quite negligible in comparison with the Global Warming in this period (~ 0.5°C) and the conclusion is that cosmic rays have a negligible effect on climate."

delady "rhj, and when you're outside on a clear, frosty morning do you notice the difference when cloud cover moves in?

I notice that it gets a bit less nippy at those times."
Certainly - I can ski in a T shirt in full sun, but when the clouds come over, it's back on with the parka. That's a huge change in energy reaching me.

Then on the other hand, in northern winter clouds warm the weather considerably. Also, clouds warm everywhere during night-time. This is because clouds also have greenhouse effect in addition to their ability to reflect sunlight. Due to greenhouse effect, high clouds cause more warming by absorbing thermal radiation than cooling by reflecting sunlight. Clouds don't just cool. Also, more clouds doesn't necessarily mean more cooling.

I think we may have yet another illustration of the difference between weather and climate here. While there seems to be some evidence indicating cosmic ray influence on cloud formation and, eventually, rainfall, the net long term effect on climate may still be negligible. In any case, it is small contributions, and if they mostly cancel out, as will very often be the case, the net effect will be close to zero. Clouds will often typically tend to have these kinds of net-zero-effects: As Ari J mentions, increasing albedo, and shielding outgoing radiation, for example.

They may, however, not cancel out, and then the net effect may become much larger. Svensmark and other proponents have, however, not been able to demonstrate such effects, and at the same time guard against competing explanations. To me, omitting or skewing the discussion of competing models (arch s, #3) is rather telling.

If we, for good measure, multiply the Erlykin estimate of temperature increase by 10, we get about as much warming as has occurred in one year recently. "Yep, it might account for one of 50 years of warming. What about the other 49?"

RHJames:
"If I simply stand outside and feel the difference between full sun, and cloud, and just feel the difference in energy reaching Earth's surface, I can't help thinking this is what it's all about."

Well if you want to appeal to the (somewhat unscientific) mind of the common person on the street, then simple thoughts like that will appeal to many. Which suggests you aren't interested in education, rather you are appealing to a persons ignorance.
In order to gain credibility skeptics need to express some consistency, that means not claiming that the climate system is to chaotic and complex on one hand, whilst at the same time suggesting simplistic answers will reveal the truth.

How often have we heard this tiresome meme in discussions of CO2 and Greenhouse-Effect, despite of strong theoretical and experimental based physics "how" the GHE works?
And now the pseudo-sceptics dismiss all of their "scepticism" and we have to dicuss a slightly correlation that "somehow" affects cloud cover!

It shows a typical pseudo-sceptic tactics: Take a point with known uncertainties, claim something nearly (but not completely) unsubstantiated, and blow it out of proportion to explain why anything but CO2 is causing Global Warming.

RHJames:
"If I simply stand outside and feel the difference between full sun, and cloud, and just feel the difference in energy reaching Earth's surface, I can't help thinking this is what it's all about."

This explains why sunny Bismark North Dakota is so much warmer in winter than cloudy Portland Oregon ...

found indications that cirrus cloud amount increases have accompanied an increase in air traffic in the 16 year period 1984–1999 ... Our mean estimate of the radiative forcing (0.03 Wm−2) is close to the number given in the IPCC (1999) (upper limit in their assessment is 0.04 Wm−2)

Wait ... these high, thin clouds are a positive forcing, slowing the escape of radiant energy to space. Airplane flight is definitely not a 'natural cycle.' Doesn't that spell anthropogenic global warming? Is that why we don't hear about contrails, but we do hear about the ephemera that is the supposed cosmic ray-climate change connection?

The CR hypothesis is so tenuous as to be irrelevant. There is no known physical process that can lead the ionized particles to the size necessary to act as condensation nuclei. The CERN experiment has yet to deliver anything useful. The so-called correlation with Forbush events seen by Svensmark over the Pacific included several days of delay, during which air masses move and all sorts of other things can happen. The CR thing literally amounts to grasping at straws.

There's this graph Richard Alley uses in his "Biggest Knob" lecture showing a great excursion of cosmic rays during the Laschamp Event, and no visible corresponding variation of temperature. From Muscheler et al. 2005
"Geomagnetic field intensity during the last 60,000 years based on 10Be and 36Cl from the Summit ice cores and 14C"

I think it's quite a striking image, and it illustrates the amplitude limitations of this possible cosmic ray influence.

#17, I would not expect much of an effect from a cloud modulation factor in the middle of a cold dry ice age. We would not expect the climate to get colder from an increase in cosmic rays since the hypothesized cooling mechanism relies on water vapor.

I don't have a scientific paper showing why that particular upward excursion did not cause "even more cooling" during an ice age, but more low clouds in an ice age may not be as cooling as more low clouds in temperate period. For one thing with less moisture in the ice age, there is less of an increase in the water cycle and thus less cooling effect than in interglacials.

But I also know that GCR are not always a well correlated factor with climate despite the papers I previously linked. Their effect is highly nonlinear and can be coincident with other solar effects that may be the cause that GCR might get credit for. The geomagnetic excursions that you point out are often terrestrial and thus remove the other solar factors from consideration. That would support more climate effect from solar factors other than the solar modulation of GCR. But that doesn't mean that GCR have no effect, just not a monotonic effect like a solar or GHG forcing.

Alexandre, thanks for the clarification. My view is also that 20th century warming cannot be attributed to GCR. GCR affects the weather in the short run which may result in cooling or warming locally (since cloud modulation effects depend on the location). I don't think it's a very strong effect relative to direct forcings like GHG, especially looking at it globally.

The atmosphere of Venus is mostly CO2 and full of dense clouds of sulphuric acid. Its surface is far hotter than Mercury which is closer to the Sun, and is due to a runaway global warming. The sceptics can choose which element is predominant in its high tempertures CO2 or cloud cover.

I just checked Web of Science and there are 167 papers referencing Svenmark's early 2000 paper. I haven't checked all of them and I'm guessing you haven't but if "Little in the scientific literature supported it then and even less does now." was true then almost all would have to have come down against Svenmark's work. If that was true it would be fair to say we wouldn't still have a debate in the science.

Just a quick check at 2010 papers finds this paper. The scientific debate seems to be more alive than you present it.

"This distribution of correlation results a possible effect to global cloud cover which is smaller than 1%."

and

"Assuming 1% effect of the cosmic rays to cloud cover, this would cause a warming of 0.002°C in global surface temperature."

Can you just confirm what you meant here? My reading is CGR may have been responsible for ~1% change in cloud cover. And this 1% change causes 0.002oC rise in temperature?

That doesn't sound right, a 1% change in clouds seems like a lot and would have a significant affect on albedo. Are you sure you got your numbers correct (I can't access the paper unfortunately). This abstract from a presentation by the authors in 2010 suggests a 0.2% CGR flux in the past 50years and a 0.01% change in clouds.

I really should have directed you to his 1997 paper "Variation of cosmic ray flux and global cloud coverage - A missing link in solar-climate relationships" which has 462 references on Web of Science and is still being actively referenced this year. I'm not sure it makes any sense to think the peer-review process would allow "this guy is wrong" to be published 462 times, or that Svensmark would be still getting grants or publishing on this work if that was the case. Common sense says the debate is still alive.

Common sense says that there is not that much of a debate if there is no physical mechanism. There is none so far by which cosmic rays can directly influence cloud formation, because it has not been shown that any kind of CCN can actually be generated by CRs.

On the other hand, CO2's radiative properties are well understood, so is their physical mechanism, with empirical measurements that agree with the theoretical calculations. It does not prevent "skeptics" to apply all sorts of doubts, even some that indicate complete diregard or lack of understanding of physics.

Cloud formation is subject to the kind of variables that "skeptics" take as an excuse all the time to claim that nothing can be asserted with any level of certainty. The lack of physical mechanism prevents any kind of theoretical prediction. The correlation claimed by Svensmark has been examined and not reproduced by other teams. Common sense says there is even less debate there than meets the eye.

Papers published that did not confirm Svensmark's hypothesis will cite him as well. I'm not so sure that 462 citations in 14 years is that remarkable.

Common sense dictates to go to the physics. If there isn't any for particle growth, this hypothesis should be given low consideration.

These results provide the most compelling evidence presented thus far of a GCR-climate relationship. From this analysis we conclude: (i) a GCR-climate relationship is governed by both the rate of GCR flux and internal precursor conditions; and (ii) it is likely that this natural forcing has not contributed significantly to recent anthropogenic temperature rises. --emphasis added

"Concerning the troposphere,it seems that there is a finite influence of CR on cloud cover at the level of f ~ 1%, a result that is mainly for clouds below about 6.5km, although when averaged over the entire atmosphere it is smaller than this."

"Disregarding the latter fact and taking an average f value of 1%, the temperature change consequent upon the changing CC given by the maximum CR change that could be allowed over the last 50 years can be calculated. Over this period the mean CR intensity appears to have fallen by less than 0.6%, using the data of Bazilevskaya et al.,(2008), so that if the conversion, ΔCR to ΔCC and there by to ΔT is known, ΔT can be calculated. Here, we adopt the conversion ΔCC = 11.3% corresponds to ΔT = 0.5°C from the work of Erlykin and Wolfendale(2010). The increase in temperature predicted is 0.002°C,..."

"There's also a possibility that the whole hypothesis rests on artifacts in cloud data."

This is going to be an issue for any work trying to find correlations between clouds and anything, including all those papers that refute Svensmark's work. The sad fact is cloud data is terrible for this sort of work.

HR#31: "an issue for any work trying to find correlations between clouds and anything, including all those papers that refute Svensmark's work."

Why? If there's no correlation between GCR flux and any observable weather effect, doesn't Svensmark's hypothesis refute itself? If there's been nothing of value from the CLOUD experiment in the last 5 years, doesn't Svensmark's hypothesis refute itself?

And of course, if there's no physical basis for it, doesn't it refute itself?

As far as I'm aware the necessary experiments have yet to be performed to answer your criticisms although there is plenty of speculation on mechanisms and even some supporting evidence (such as much of Harrison's work). Applied science is never based on one piece of evidence and an idea is never dismissed because one piece of the puzzle still eludes us. I guess you fully support the continued investigation of this problem?

"I'm not so sure that 462 citations in 14 years is that remarkable." That's funny!

29 muoncounter

This obscure paper from the Indian Academy of Science Journal seems to suggest the impact on global temperature is very different to the paper Ari has presented. Very few scientists have bothered to quantify the impact of CGR on global temperatures, I'm not sure that Svensmark has even bothered to do this.

I think my position on this science is much like the rest of climate science. Uncertainty in the science rules, which suggests further investigation before firm conclusions can be made. Those that seem to want to kill the CGR/climate connection are as ideologically motivated as the worst sceptics. And the IPCC position on this issue can only have the effect of strangling research on this subject.

"Those that seem to want to kill the CGR/climate connection are as ideologically motivated as the worst sceptics" Wow, how did you get there? Who is trying to kill the connection - if so, why did CLOUD even get built? What are the 462 citations you claimed in #27? How does that in any way suggest anyone is trying to kill the connection?

Please do not make such accusations without any trace of substantiation.

No, the problem with the GCR/climate connection is that no one can substantiate it and that 'skeptics' are desperately trying to cling to it. Do you think they are ideologically motivated?

Yes Muon, funny how the "skeptics" play up the so-called "uncertainty" surrounding Anthropogenic Warming, yet are very quick to promote any other potential source of warming-regardless of how *weak* the foundation for it is. Anyone would think they were trying to "kill the CO2/climate connection".

#33 HR.
I suggest you contemplate the difference between "killing the connection" and quantifying it. I think this is quite analogous to what I often experience in medicine/health studies, where we may identify covariates in regression models that may be highly significant statistically, because we have so many subjects and good precision. BUT only account for, say 0.5% or less of the total variation - significant but, in the majority of cases, not relevant.

If all the self-declared "skeptics" could make it a routine exercise to estimate the likely net effect of what they are looking at, and from there do a quick judgment of relevance, the discussion would benefit a lot. And, of course, one must take all the available data into consideration when estimating.

By all indications, the eventual climate effect of GCR is very small, and if it were very strong, this would normally have been demonstrated or strongly hinted at by now.

HR:
"Applied science is never based on one piece of evidence and an idea is never dismissed because one piece of the puzzle still eludes us."
Curiously, that is exactly what some regular skeptic contributors have suggested when they protested against the larger picture type of concept. I will make sure to cite you when running into attempts to do exactly that by so-called climate skeptics. I know it won't be long until it happens. In this case, it can be reasonably argued that the physical mechanism is more than just "one piece of the puzzle."

"that's funny." Why?

"The necessary experiments" is exactly what CERN's CLOUD program is all about. You know, that darn government-run, public-money scientific research thing. So far the only truly clear result is that the chamber walls overwhelm whatever other effect could possibly exist. Now, whomever is running the program must manage to get money to continue it. It may be decided that CERN's priorities are elsewhere and I wouldn't venture to emit a judgement on the issue if it happens. I'm sure there are countless people better qualified than me to say how CERN should allocate resources.

However, your snark is unwarranted. I'm all for understanding nature, even that part.

Don’t quote me out of context. Why would you do such a thing? (Rhetorical question- just think about it).

What I said was: “I admit that it is an interesting hypothesis, but only folks highly selectively biased by personal belief could possibly believe that galactic cosmic rays have a climate effect greater than that of the change we have seen in atmospheric CO2 levels. Little in the scientific literature supported it then and even less does now.”

I never said there was little evidence to support the fact that CR may affect cloud cover. Indeed there is. What is lacking is substantial evidence that the changes in cloud cover are significant enough to affect climate to a degree greater than (anthropoid changes in) CO2 does.

I thought this summary was a good one when it came out 2006 at the start of the CERN CLOUD experiments.

There is evidence that GCRs can trigger particles that can act as cloud condensation nuclei. There is little evidence that this has any influence of our climate, and significant more (and more convincing) evidence that it does not.

arch #43: "significant more (and more convincing) evidence that it does not."

I suppose that depends on what it takes to convince. We're now far enough from the solar min to make some observations. From Mewaldt et al 2010:

In the energy interval from ~70 to ~450 MeV nucleon–1, near the peak in the near-Earth cosmic-ray spectrum, the measured intensities of major species from C to Fe were each 20%-26% greater in late 2009 than in the 1997-1998 minimum and previous solar minima of the space age (1957-1997) ... Cosmic-ray intensity variations at 1 AU are found to lag IMF variations by 2-3 solar rotations

That rotation period is 27 1/4 days, as viewed from earth. So the maximum lag, if any, is less than 90 days.

According to the model, these elevated GCR intensities should make more clouds. Do 2009-10 or 1997-98 appear to be cloud-cooled? From tamino's adjusted GISS temperature graph ("we remove the el Nino, volcanic, and annual-cycle signals we have what we’ve called the adjusted GISS data"),

To summarize (with apologies to Sondheim),And where are the clouds?
There ought to be clouds.
Well, maybe next year.

Further correction Arch (sorry, it needs to be said): The particles generated by GCR are too small to act as CCN. A plausible mechanism for particle growth is the physical process lacking to give weight to the hypothesis.

"At issue is the fact that so many other particles can form CCNs that the question was whether or not the additional (GCR generated) ones were significant"

That is indeed a big question. Oceanic air, although less rich in CCN than that over land, gets a lot of ions from sea spray, and these tend to be in already big clusters.

Useable size for CCN is about 30nm diameter. The particles created by GCRs are about 0.5.This covers some of the processes necessary for CRs to have any effect at all. As you will see, it is rather convoluted. It is far from obvious whether or not it can be a factor at all in either weather or climate.

Furthermore, it has been shown already that CCN size is a more important factor than chemistry.

#44, muoncounter, why are you looking for clouds in a temperature graph? Cloud fraction at Barrow AK dropped from 1998 to 2008 http://www.agu.org/journals/jd/jd1017/2009JD013489/2009JD013489.pdf I would expect local areas of the globe to react differently to GCR regarding clouds. Whether that results in a GAT change is a super stretch.

Isn't that the entire point? These GCR-stimulated clouds are supposed to cool; during GCR lows (solar highs), the lack of these clouds is supposed to be an agent of warming. So why not look in a temperature graph? Read the first and last sentences of the post.

The cloud fractions (CFs) derived from ARM radar-lidar and ceilometer measurements increase significantly from March to May (0.57→0.84), remain relatively high (∼0.80–0.9) from May to October, and then decrease from November to the following March (0.8→0.57), having an annual average of 0.76.

Seems to be a seasonal effect, which isn't a hallmark of GCRs.

The sensitivity study has shown that LW CRFs increase with increasing cloud fraction, liquid water path, and radiating temperature with high positive correlations (0.8–0.9). Negative correlations are found for SW CRFs, but a strong positive correlation between SW CRF and surface albedo exists.
CRF=cloud radiative forcing; more clouds -> greater retention of LW energy.